Suspected Arsonists Knew Exactly Where to Strike Paris Railways to Heighten

France took virtually every precaution to make sure that its capital would be secure when all eyes were on the city for the opening ceremony of the Paris Olympics.

But the saboteurs who disrupted travel for hundreds of thousands on Friday did so by striking far outside the capital, targeting a rail network so vast, experts say, that it is impossible to secure every foot of it. And they knew exactly where to strike to cause maximum chaos.

“The S.N.C.F. faces attacks on its network every year, every month even,” said Julien Joly, a transportation expert at the Wavestone consulting firm, using the abbreviation for France’s national railway company.

He added: “But never in these proportions, and never in such a coordinated way. Unfortunately the network is so vast, you can’t ensure a 24-hour security presence everywhere.”

France has a very dense train network, with 28,000 kilometers of tracks (about 17,400 miles) used by 15,000 trains every day. Much of it is separate from the high-speed train network.

But the TGVs, as the high-speed trains are known, whisk thousands of passengers across the country daily, and are one of France’s most prized national infrastructures — a symbol of its technical know-how.

Arnaud Aymé, a transportation expert at the Sia Partners consulting firm, said that the infrastructure that was targeted — signal stations and cables — were often protected with alarms, cameras, fencing and barbed wire, and were not always easily accessible.

But train networks, because they are expansive but also limited by nature — a technical failure on one track creates an immediate bottleneck — are hard to make failproof.

“Railway networks are particularly open,” Mr. Aymé said, vulnerable to a fallen tree, an unlucky lightning strike on an electrical substation or a malicious act.

Any disruptions have “cascading effects,” he said. “Even if the network is repaired, it takes time for everything to get back in order,” he added.

Railway networks face thousands of criminal acts every year, experts say: People steal electrical cables to sell copper wire on the black market or carry out petty acts of vandalism.

But the level of coordinated and simultaneous sabotage that happened Friday is extremely rare, experts say. The S.N.C.F. said that it detected the arson around 4 a.m. and quickly intervened alongside firefighters and police.

French prosecutors have opened a criminal investigation. They say there is no doubt that the fires were intentionally set, but have said that it is too early to determine who the suspects are and what had motivated them.

It was also unclear if the aim was to disrupt the Olympics directly, or merely to embarrass the French authorities as the country had large-scale international attention. Experts noted that sabotage on the Paris commuter lines — which are expected to ferry hundreds of thousands of Olympic visitors — would have been more directly devastating to the Games.

None of the targets are close to the French capital. The S.N.C.F. said that arsonists successfully targeted infrastructure in Courtalain, southwest of Paris; Pagny-sur-Moselle, to the east; and Croisilles, to the north. All…